Several types of computer memory have in common the feature that they utilize "hard" disks which are comprised of a base or substrate material, such as aluminum, glass, plastic or ceramic, upon which are thin layers of various material including materials which are magnetic or optically active, plus layers of materials which protect the active layer(s) from corrosion, or mechanical damage. Such magnetic, optical or magneto-optical disks may also be referred to as media. An example of such media is the magnetic disks which are used in hard disk drives.
In typical applications a thin polished or textured aluminum disk is used as the substrate. Various thin films are deposited by vacuum sputtering upon this substrate. Typically an underlayer or underlayers 200 to 1000 Angstroms (.ANG.) thick is first deposited, then a magnetic layer, 100 to 600 .ANG. thick is deposited over this underlayer and then a protective thin layer is deposited over the magnetic layer. This protective layer is typically 50 to 150 .ANG. of carbon.
The carbon layer serves several important functions. It protects the magnetic layer against corrosion damage from water vapor or other atmospheric contaminants. It also protects the magnetic layer from mechanical damage, which could occur as a result of contact of the disk with the magnetic read-write head. The head normally "flies" above, but very close to, the surface of the disk. However, when the drive is not in operation the head may rest on the disk, or it may contact the disk inadvertently as a result of shock, vibration or malfunction when the drive is operating. To obtain maximum magnetic signal strength it is desirable to "fly" the head as close as possible to the magnetic layer. Spacing of 1 micro-inch (250 .ANG.) or less is typical in current drives. Such close spacing can result in relatively frequent contacts between the head and the disk, and is sometimes referred to as semi-contact recording. If frictional forces are high, such contacts can destroy parts of the disk surface.
Both the disk surface and the head have relatively flat, smooth surfaces. As a result, there can be substantial friction between them when they are in contact. This is particularly significant when the head has been resting for some time against the disk. In this case the starting friction or "stiction" between the head and the disk can be so great that the motor seeking to start the disk rotating could have insufficient force to do so, or if it does, the carbon protective coating of the disk may be damaged in the process.
To minimize these problems, disk manufacturers typically apply a thin (10-15 .ANG.) film of a lubricant over the carbon topcoat. The commonly used method of applying the lubricant is described in U.S. Pat. No. 5,232,503 and is as follows:
1. After a carbon topcoat is applied in the vacuum sputtering system, the disks are loaded in a cassette, which typically holds 25 such disks. The cassettes exit the vacuum system into atmosphere.
1. The cassettes then proceed to the lubricant system where, typically, the disks are removed from the cassette as a batch and lowered into a tank containing a mixture of lubricant and solvent. This mixture is then withdrawn from the tank in a controlled manner to leave a uniform thin coating of lubricant on the disks.
2. The lubricant-coated disks are returned to cassettes where they proceed to further processing and/or testing.
In addition to being very thin (10 to 15 .ANG.) and very uniform (to within 2 .ANG.), the lubricant layer must satisfy a number of requirements. Obviously, it must be adequately lubricious to reduce friction and stiction between the head and the disk. It must also bond sufficiently well to the carbon layer to resist evaporation and the centrifugal forces generated by the disk, which rotates at speeds up to 10,000 rpm, so that it remains largely in place and is not spun off. At the same time it must be able to flow somewhat so as to be able to recoat areas of the disk where the lubricant may have been displaced as a result of contact with the head.
Obviously, achieving the proper balance between fixity and mobility is difficult and requires careful control of the properties of the lubricant and of the process for applying it. At present, a difficulty arises since the fresh carbon surface is reactive and begins to contaminate as soon as it is exposed to atmosphere. This leads to varying times of such exposure, since the lubrication process systems handle disks in the batch mode. The lubricant, which is designed to bond to the surface, does so non-uniformly since the contaminated surface is not uniform and surface contamination increases as the disks remain exposed to atmosphere. Such contamination makes it difficult to keep the lubrication process under control, which in turn affects disk drive reliability. Also solvents present and used in the gravity lube process are considered air pollutants, requiring that the present lubrication systems be built with elaborate and expensive means to minimize solvent escape.
Although the construction of optical and magneto-optical disks is not the same as conventional magnetic hard disks, they also have the characteristic that the head flies very close to the disk surface and therefore occasional head-disk contact occurs. To minimize potential damage to both the head and the disk a top lubrication layer is frequently used on such disks also, with similar problems of process and environmental control.
Disks are manufactured at a reasonably rapid rate using serial feed and processing. Yet most of the current processes for lubricating disks use batch processing. Manufacturing processes combining serial and batch processing, although possible, make disk production most complex. The instant invention permits continuous serial feed as to simplify manufacturing techniques.